Nitric oxide (NO) production from inducible NO synthase (iNOS) plays an important role in the inflammatory response of cardiac transplant rejection. While iNOS is regulated transcriptionally, the role of posttranslational regulation of iNOS activity in acute rejection is unknown. HYPOTHESIS: Posttranslational factors alter the production of NO from iNOS and determine the downstream actions of NO in acute cardiac rejection. METHODS: Lewis (isograft) or Wistar-Furth (allograft) donor hearts will be transplanted into Lewis recipient rats. Isolated cytokine-stimulated cardiac myocytes(CM) will also be used. Gene expression will be determined by Western blot and RT-PCR. BH4 and NO synthesis will be determined by HPLC, NO analyzer or electron paramagnetic resonance (EPR) spectroscopy. Uncoupling of NO vs. superoxide production by varying co-factor synthesis (including GTP cyclohydrolase I overexpression), iNOS dimerization and nitrotyrosine formation will be examined by state-of-the-art biophysical and biochemical techniques including: get filtration with Western blot, immunohistochemistry, ELISA, EPR and fluorescence spectrosocpy, chemiluminescence. AIM#1: BH4 regulates NO production from iNOS in cytokine-activated CM and in CM after alloimmune activation. AIM #2: 6H4 regulates superoxide production from iNOS in cytokine-actived CM and in CM after alloimmune activation. AIM #3: Alteration in NO and superoxide production in cytokine-activated CM and in CM after alloimmune activation has downstream effects on biological markers of lipid peroxidation, protein oxidation and protein nitration. Findings from this study may provide unique strategies to protect against oxidate and nitrative injury in acute cardiac rejection. Public Health Statement: The loss of cardiac muscle cells is a significant problem in human cardiac transplants that may contribute to poor heart function. Our studies identify a potential molecular problem intrinsic to cardiac cells that predisposes to cell death and injury. A better understanding of this molecular process may lead to better strategies to prevent injury cardiac cells in these patients.